1. Field of the Invention
The present invention relates to a duplex imaging apparatus for forming images on the two faces of a sheet and, more particularly, to improvements in the duplex imaging apparatus of the mode in which a carrier has a plurality of image carrying regions to transfer an unfixed image on the image carrier to a sheet and to fix the unfixed images, as formed consecutively on the two faces of the sheet, individually by a fixing unit.
Here, the image carrier is defined to include not only an image forming carrier such as a photosensitive member but also an intermediate transferor. Therefore, the image carrier naturally includes the mode in which only the intermediate transferor has a plurality of image carrying regions, in the duplex imaging apparatus having the intermediate transferor, for example.
2. Description of the Related Art
The imaging apparatus of the prior art will be exemplified by an intermediate transfer type imaging apparatus. This imaging apparatus is provided with developers of individual color components of black (Bk), yellow (Y), magenta (M) and cyan (C), for example, around a latent image carrier such as a photosensitive drum, and a belt-shaped intermediate transferor, for example, is arranged to face the latent image carrier. The unfixed toner images of the individual color components, as formed on the latent image carrier at each turn of the latent image carrier, are primarily transferred in a consecutive manner to the intermediate transferor, and the synthesized primary transfer images, as superposed on the intermediate transferor, are then secondarily transferred to a sheet such as a form or an OHP sheet thereby to form a desired image on the sheet (as disclosed in Unexamined Published Japanese Patent Application No. 5-323704, for example).
This type is advantageous in that the disturbance of the image and the color shift at the time of a multiplex transfer can be effectively prevented without considering the thickness or surface characteristics of the sheet or the conveyance characteristics of the sheet to the latent image carrier, because the synthesized toners image having been multiplexly transferred to the intermediate transferor are transferred altogether to the sheet.
Of the imaging apparatus of this kind, moreover, the duplex imaging apparatus for forming images on the two faces of the sheet performs a series of processes to fix the unfixed toner image, as transferred to one side (or surface) of the sheet, by a fixing unit at the time of selecting the duplex mode, for example, to convey the fixed toner image again to the secondary transfer portion after the sheet is inverted through a sheet returning inversion mechanism, to transfer the synthesized primary transfer image or a sheet back image, as formed on the intermediate transferor, secondarily to the other side (or back) of the sheet, and to fix the secondarily transferred image in the fixing unit.
On the other hand, the fixing unit is equipped with a pair of fixing rolls (i.e., a heat fixing roll and a pressure fixing roll to be brought into pressure contact with the heat fixing roll) to turn in contact with each other, for example. The unfixed toner on the sheet is fixed by passing the sheet through the nip region of the fixing rolls. From the standpoint of preventing the so-called "offset phenomenon" in which the toner image on the sheet transits toward the fixing rolls, there is usually adopted a method of feeding the individual fixing rolls with a releasing agent or oil (e.g., silicone oil).
At this time, it is arbitrary to add a releasing agent feeder to each of the paired fixing rolls. From the standpoint of simplifying the apparatus construction, however, there is frequently adopted a method of arranging a releasing agent feeder only on the side of the heat fixing roll to feed the heat fixing roll directly with the releasing agent and the pressure fixing roll indirectly through the surface of the heat fixing roll with the releasing agent.
On the other hand, the amount of the releasing agent to be fed could be controlled according to the amount of the releasing agent applied to the fixing roll. From the standpoint of simplifying the apparatus construction, however, there is adopted a method of feeding a constant amount easily.
As a method of enhancing the productivity in the aforementioned duplex imaging apparatus, moreover, there has already been proposed a method by which the individual images can be individually transferred to the sheet by releasing a plurality of image carrying regions on an intermediate transferor to retain a plurality of images on the intermediate transferor in a series of imaging cycles till the transfer to the sheet (as disclosed in Unexamined Published Japanese Patent Application No. 7-306618, for example).
Here, the intermediate transfer type duplex imaging apparatus of this kind has been found to have a technical problem that the halftone image of a wide range in the simplex mode is liable, if taken after consecutive fifty runs of an identical image in the automatic duplex mode, to have a lower density than the remaining regions thereby to appear as a residual image (as will be called the "oil ghost phenomenon", if necessary).
We have investigated the causes for the aforementioned oil ghost phenomenon and have reached the following conclusions.
In the duplex imaging apparatus including a fixing unit in which a constant feed type releasing agent feeder is attached only to the side of the heat fixing roll, for example, it is assumed that the consecutive runs of an identical image are made in the automatic duplex mode.
At this time, as shown in FIG. 31A, a sheet 300 having an unfixed image formed on its one side (or surface) is guided through a fixing unit 301. Since a releasing agent 303 is also fed to the side of a pressure fixing roll 302 of the fixing unit 301, it occurs that the releasing agent 303 sticks to the substantially whole area of the other side (or back) of the sheet 300.
At the transfer time of the sheet 300 to the back, moreover, when the sheet 300 having one fixed side passes again through the secondary transfer portion, the releasing agent 303 having stuck to the back of the sheet 300 will transit to an intermediate transferor 304, as shown in FIG. 31B, in a manner to correspond to a non-image portion 305 [the region of the sheet 300 excepting an unfixed image (or image portion) 306 for the back on the intermediate transferor].
When the identical image is to be subjected to considerably consecutive runs, the releasing agent 303 is hardly fed to the portion of the image portion 306 at the identical portion of the intermediate transferor 304 but will sequentially transit only to the portion corresponding to the non-image portion 305, so that the transfer efficiency of the non-image portion 305 rises far extremely than the image portion 306.
If the wide range halftone image is taken in this state in the simplex mode, as shown in FIG. 31C, the portion corresponding to the image portion 306 in the automatic duplex mode has a lower transfer efficiency than the other portion [i.e., the portion corresponding to the non-image portion 305 in the automatic duplex mode], there seems to appear the residual image (or the negative ghost) in which a halftone image 307 having a thin portion corresponding to the image portion 306 turns into a halftone image 308 having a thick portion corresponding to the non-image portion 305.
Especially in the duplex imaging apparatus including an intermediate transferor having a plurality of image carrying regions, there arises a situation in which the images in the individual image carrying regions on the intermediate transferor are consecutively transferred to the plural sheets so that the individual sheets pass consecutively through the fixing unit.
At this time, the fixing unit is indispensably required to perform the fixing process without any offset even under the condition in which the plural sheets consecutively pass. It is, therefore, necessary to set the amount of releasing agent to be fed more than that of the mode of one image carrying region. As a result, the aforementioned oil ghost phenomenon is liable to appear more prominently.
In the duplex imaging apparatus thus far described, on the other hand, the conveyance span of the sheets to pass through the fixing unit becomes longer when the full color mode for forming the full color image is selected than when the ordinary monotone mode is selected. As a result, the amount of releasing agent to transit from the heat fixing roll surf ace of the fixing unit to the pressure fixing roll becomes more so that the aforementioned oil ghost phenomenon becomes liable to appear more prominently.
Here, the technical problems thus far described should not be limited to the intermediate transfer type duplex imaging apparatus but can likewise occur in the direct transfer type duplex imaging apparatus in which the images are transferred directly to the sheets from a latent image carrier such as a photosensitive belt.